The present invention relates to a charge and discharge control method for equalizing at an intermediate state the states of charge (SOC) of each of a plurality of cells which together constitute a battery pack, in which the cells that are individually constructed as rechargeable battery are connected in series.
State of charge (SOC) of a rechargeable battery is defined as the proportion of the quantity of electricity that is charged in the rechargeable battery relative to the electrical capacity of the battery. The SOC varies depending on the temperature of the battery or the variance of the characteristics of each of the individual cells. Therefore, in the case where a plurality of cells are connected in series to constitute a battery pack, there will be differences between the SOCs of each of the cells.
As a technique for equalizing the SOCs of each individual cell that constitutes a battery pack, particularly for a rechargeable battery using lead or nickel in which an electrolyte of aqueous solution type is used, a method of equalizing the differential between the cells with low SOCs and those with high SOCs by charging the entire battery pack in a state of overcharge is adopted in prior art. In the case of rechargeable battery using lithium which cannot be overcharged, a charging circuit is used for controlling charging of each of the cells.
However, since equalization of the SOC of each cell by overcharging leads to deterioration of the rechargeable battery, there is a limit to the number of conducting overcharging, and therefore constant equalization of SOCs cannot be achieved by this method. The method of controlling the charging of each of the cells that constitute the battery pack is not practical, since it entails complicated construction of charging circuit and its control circuit, which causes considerable raise in cost. In the application wherein the rechargeable battery is recharged by recovering energy during idling of an internal combustion engine, braking, or coasting, such as in a hybrid vehicle which uses an electric motor and an internal combustion engine in combination as power sources, or in a hybrid bicycle wherein the pedaling action by human power is supplemented by a power from an electric motor, it is necessary to maintain the SOCs of the rechargeable batteries at an intermediate level; thus equalization of SOCs is essential. In such applications the method by overcharging as mentioned above cannot be adopted, and the method of controlling the charging of individual cells is not suitable either, because of its high device cost and difficulty in management of charging.
An object of the present invention is to provide a method of controlling charging and discharging of a rechargeable battery for equalizing the SOCs of each of the cells that constitute a battery pack, without using a method that leads to deterioration of the battery, or without increasing the device cost.
To achieve the above object, the present invention provides a method of controlling charge and discharge of a rechargeable battery for a battery pack wherein a plurality of cells, individually constructed as rechargeable battery, are connected in series, for equalizing the state of charge of each of the cells, characterized in that: a charging efficiency of the rechargeable battery corresponding to a target value of SOC is obtained, and the battery pack is charged and discharged repeatedly such that the proportion of a discharging amount with respect to a charging amount is equal to the obtained charging efficiency.
Charging efficiency is an index of how much quantity of electricity used for charging is stored in the rechargeable battery. The charging efficiency is high when the SOC of the rechargeable battery is low, whereas it is low when the SOC is high. Therefore, if the battery pack which is in a state wherein each of the serially connected cells has different SOCs is charged and discharged repeatedly such that the proportion of the discharge amount with respect to the charge amount is equal to the charging efficiency corresponding to the target value of SOC, when the battery pack is charged, the cells with an SOC lower than the target SOC will be charged with more electricity by the amount of differential in the charging efficiency, while the cells having an SOC higher than the target SOC will be charged with less electricity by the amount of differential in the charging efficiency. On the other hand, when the battery pack is discharged, all of the cells are uniformly discharged by the amount obtained by multiplying the charging amount by the charging efficiency. Accordingly, the cells having an SOC lower than the target SOC will have more stored charge by the charged electricity corresponding to the differential in the charging efficiency, while the cells having an SOC higher than the target SOC will have less stored charge by the amount of electricity corresponding to the differential in the charging efficiency. By repeating charge and discharge as described above, the SOC of the cells that was lower than the target SOC increases gradually toward the target level, while the SOC of the cells that was higher than the target SOC decreases gradually toward the target level. Accordingly, by continuing such cycles of charging and discharging, the SOCs of each of the cells can be equalized.